A proposed high-gain free-electron laser (FEL) amplifier, conceived primarily for use in a maritime environment, would be capable of average output power of the order of a megawatt at one or more of several infrared wavelengths (1.045, 1.625, or 2.141 μm) for which propagation through air is minimally adversely affected by water vapor and aerosols. The conceptual design of this FEL amplifier provides for a combination of conventional and unconventional characteristics that, taken together, would be advantageous in the intended power and wavelength regime.

The High-Gain FEL Amplifier would accept input from a low-average-power FEL. The optical beam would be optically guided in the wiggler and optically pinched at the output end of the wiggler. Pinching of the optical beam would reduce its Rayleigh length and cause it to undergo rapid diffraction upon leaving the wiggler. Orienting the first relay mirror for grazing incidence would prevent damage to the mirror by spreading the beam power over a large mirror area.
The proposed FEL amplifier (see figure) would feature refractive optical guiding in the wiggler, with pinching of the electron and optical beams at the output end of the wiggler. The electron beam would be driven by a radio-frequency linear accelerator (not shown in the figure).

The gain length, efficiency, electronpulse slippage, and distance between the wiggler and the first relay mirror have been calculated for the conceptual design. Of particular concern in the design is the overall length of the amplifier optical system; that is, the length of the wiggler and the distance from the wiggler to the first relay mirror. The wiggler would be about 1.5 m long. The distance from the wiggler to the first relay mirror would be chosen so that the threshold intensity for damage to the mirror would not be exceeded, as described next.

The focusing of the electron beam and the desired consequent pinching of the optical beam at the output end of the wiggler could be effected by application of an externally generated magnetic field or by utilizing the betatron oscillation of the electron-beam envelope. Pinching of the optical beam would reduce its Rayleigh length, thereby making it possible to put the first relay mirror closer to the wiggler without exceeding the damage threshold. The mirror would be oriented for grazing incidence to reduce the intensity at incidence and thereby increase the damage threshold. The combination of pinching the optical beam and grazing incidence would enable placement of the first relay mirror at or perhaps closer than a distance of 3μm from the output end of the wiggler.

The FEL efficiency could be increased by tapering the wiggler. Alternatively, if using a uniform wiggler, enhanced efficiency could be obtained through frequency detuning of the input oscillator signal. Electron-pulse slippage, which could be large enough to limit the FEL interaction length in a low-gain design, has been shown to be small enough not to be of concern in the present high-gain design conceptual design. In one example design calculation, it was found that for an electron-beam current of 1 kA at an electron kinetic energy of 81 MeV, a wiggler magnetic flux density of 5 kG (0.5 T), frequency detuning of -1.5 percent, and linear-accelerator duty factor of 1.5×10-3, the FEL power-gain length would be 14 cm, the intrinsic efficiency would be about 1.2 percent, the peak optical output power would be 1 GW, and the average optical output would be 1.5 MW.

This work was done by Phillip Sprangle and Joseph Peñano of the Naval Research Laboratory, and Bahman Hafizi of Icarus Research, Inc.

NRL-0025


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Compact Megawatt Infrared Free-Electron Laser Amplifier

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This article first appeared in the December, 2007 issue of Defense Tech Briefs Magazine.

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